Engineering thermoplastics are a group of polymers that possess a balance of properties comprising strength, stiffness, impact resistance, and long term dimensional stability that make them useful as structural materials. Engineering thermoplastics are especially attractive as replacements for metals because of the reduction in weight that can often be achieved as, for example, in automotive applications.
For a particular application, a single plastic may not offer the combination of properties desired and, therefore, means to correct this deficiency are of interest. One particularly appealing route is through blending together two or more polymers (which individually have the properties sought) to give a material with the desired combination of properties. This approach has been successful in limited cases such as in the improvement of impact resistance for plastic, e.g. polystyrene, polypropylene, poly(vinyl chloride), etc., using special blending procedures or additives for this purpose. However, in general, blending of plastics has not been a successful route to enable one to combine into a single material the desirable individual characteristics of two or more polymers. Instead, it is often been found that such blending results in combining the worst features of each with the result being a material of such poor properties as not to be of any practical or commercial value. The reasons for this failure are rather well understood and stem in part from the fact that thermodynamics teaches that most combinations of polymer pairs are not miscible, although a number of notable exceptions are known. More importantly, most polymers adhere poorly to one another. As a result, the interfaces between component domains (a result of their immiscibility) represent areas of severe weakness in blends and, therefore, provide natural flaws and cracks which result in facile mechanical failure. Because of this, most polymer pairs are said to be "incompatible". In some instances the term compatibility is used synonymously with miscibility, however, compatibility is used here in a more general way that describes the ability to combine two polymers together for beneficial results and may or may not connote miscibility.
One method which may be used to circumvent this problem is polymer blends is to "compatibilize" the two polymers by blending in a third component, often referred to as a "compatibilizing agent", that possesses a dual solubility nature for the two polymers to be blended. Examples of this third component most typically are obtained in block or graft copolymers. As a result of this characteristic, this agent locates at the interface between components and greatly improves interphase adhesion and therefore increases stability to gross phase separation.
The present invention covers a means to stabilize multipolymer blends that is independent of the prior art compatibilizing process and is not restricted to the necessity for restrictive dual solubility characteristics. The materials used for this purpose are special block copolymers capable of thermally reversible self-cross-linking. Their action in the present invention is not that visualized by the usual compatibilizing concept as evidenced by the general ability of these materials to perform similarly for a wide range of blend components which do not conform to the solubility requirements of the previous concept.